Means for amplifying electrical currents of audible frequencies



May 20, 1930. w. VAN B. ROBERTS 1,759,631

MEANS FOR AMPLIFYING ELECTRICAL CURRENTS OF AUDIBLE FREQUENCIES FiledFeb. 2a, 19 6 l l l l l l l INVENTOR WALTER VAN B. ROBERTS ORNEYPatented May 20, 1930 UNITED STATES PATENT ounce WALTER VAN B. ROBERTS,OF BRONXVILLE, NEW YORK, .ASSIGNOIE. TC RADIO CORPORA- TION OF AMERICA,A CORPORATION OF DELAWARE MEANS FOR AMPLIFYING ELECTRICAL CURRENTS OFAUDIBLE FREQUENCIES Application filed February 26, 1926. Serial No.90,771.

It is well known in the art that in an audio frequency amplifier a gainin quality is secured if the grids of the amplifying tubes are kept at apotential more negative than that of the negative end of the filament.The present invention relates to a novel means for obtaining thisnegative grid potential. Other purposes and objects may be observed inconnection with the annexed specification, claims and drawings in whichFig. 1 shows a two stage amplifier, and

Fig. 2 shows a multi-stage amplifier.

A condenser (1) of lar e capacity (I have found that one microfara is asuitable value) is connected between the common lead (3) to the negativeends of the filaments and the common grid return wire (4) from thesecondaries of the transformers. With this arrangement a negative biasof the grids is produced b the signals themselves.

The theory of the operation of this device is as follows -The audiofrequency currents to be amplified are caused to flow through theprimary winding 5 of the first transformer. The successive positivehalves of the alternations of potential of the grid end of the secondarywinding 6 of the last transformer draw electrons to the grid 9 untilcondenser 1 is so charged that both transformer secondaries 2 and 6 andboth grids 8 and 9 are at a negative potential with respect to thenegative filament lead 3 and this negative potential is approximately eual to the peak voltage of the secondary 6 of t e last transformer.

If no leakage of charge occurs the grids remain at this negativepotential unless stronger signals come along and still further increasethe negative potential. In actual practice a permanent negative biasindependent of subsequent signals is not secured because no insulationis perfect and even in the best evacuated tubes there are always somepositive ions which are attracted to the grids and gradually neutralizethe charge on the condenser. Hence, the negative grid potential tendsmore or less slowly to diminish toward equality with the peak volta eapplied. The important fact however is that the negative grid potentialnever falls appreciably below the peak applied voltage at an time. Theaction of a grid bias battery di ers from the self biasing actiondescribed above in that the peak applied voltage may rise far above thevoltage of a biasing battery and remain so for any lengthof time.

It is sometimes desirable to provide a discharge path for the condensersupplementary to that afforded by insulation imperfection and positiveions in the tube, so that if a strong crash of static or other voltageof much greater than normal intensity causes the potential of the gridsto assume too great a negative value, the excess charge will leak off ina reasonable length of time. I have found that a resistance of one ortwo million ohms or more is suitable for this purpose and that it may beconnected between grid and negative filament terminal, or in parallelwith the condenser as shown in Fig. 1 where the connections to theresistance 10 are shown in dotted lines.

I have explained the invention as applied to a two stage audio frequencyamplifier where it serves to maintain equality of bias on the two grids.The scope of the invention is not limited-to this case however, as it isequally applicable to any amplifier of more or less than two stages.Furthermore, a multistage amplifier may be considered to be a cascadedcombination of amplifiers each of fewer stages, and each of these mayhave a self biasin device. The biasing potentials developed by suchsuccessive devices will be increasingly great because the signal isincreasingly strong in successive sections of the amplifier.

The above described method of self biasing may be advanta eouslycombined with the battery method 0 grid biasing, by properly inserting agrid battery in series with the self biasing device, so that the gridpotential never falls appreciably below the peak value of the impressedvoltage nor below the battery voltage.

Fig. 2 shows a multistage amplifier emresistances 10 and 12 that 'may bechosen to have any suitable value.

A convenient arrangement in practice, especially when the grid potentialof all the tubes in the amplifier is derived from a single device, is toprovide binding posts 13 and 14 between which the condenser can beconnected as shown in Fig. 1. These same posts are then available forconnecting to a bias battery when desired. Or, between them a biasbattery and condenser provided with a suitable high resistance dischargepath may be connected in series.

Having thus described my invention, I do 15 not wish to be limited bythe exact construction shown but only to the extent of the followingclaims: 7

1. In combination, a series of amplifiers, each having anode, cathodeand control elecg0 trodes, input and output circuits, the cathodes beinginductively connected, the control'electrodes being connected to theirrespective input circuits, the anodes being connected to theirrespective output circuits, and another connection from the inputcircuits to, the cathodes including a bias condenser and a bias batteryfor maintaining the said control electrodes at a biasing potential whichis substantially equal to the peak value of the signal ac voltageapplied to the control electrodes, and

above the battery potential.

2. In combination in an audio amplifier having anode, cathode andcontrol electrodes, a series connection between the control electrodeand the cathode including a bias condenser and a source of constant biaspotential; for maintaining the bias potential of the control electrodeat least as great as the peak value of signal voltage applied to saidcontrol electrode at all times, and above the potential of said sourceof constant bias potential.

WALTER VAN B. ROBERTS.

